EP 2 667 097 A1 discloses a method for operating a gas burner appliance. During burner-on phases, a defined gas/air mixture having a defined mixing ratio of gas and air is provided to a burner chamber of the gas burner appliance for combusting the defined gas/air mixture within the burner chamber. The defined gas/air mixture is provided by a mixing device mixing an air flow provided by an air duct with a gas flow provided by a gas duct. The air flow flowing through the air duct is provided by fan in such a way that the fan speed of the fan depends on a desired burner load of the gas burner appliance, wherein the fan speed range of the fan defines a so-called modulation range of the gas burner appliance.
According to EP 2 667 097 A1, the defined mixing ratio of gas and air of the gas/air mixture is kept constant over the entire modulation range of the gas burner appliance by a pneumatic controller. The pneumatic controller uses a pressure difference between the gas pressure of the gas flow in the gas pipe and a reference pressure, wherein either the air pressure of the air flow in the air duct or the ambient pressure is used as reference pressure, and wherein the pressure difference between the gas pressure of the gas flow in the gas pipe and the reference pressure is determined and controlled pneumatically. The combustion quality is monitored on basis of a signal provided by a combustion quality sensor like a flame ionization sensor.
According to EP 2 667 097 A1, during burner-on phases of the gas burner appliance, the mixing ratio of the gas/air mixture can be calibrated to different gas qualities on basis of the signal provided by the flame ionization sensor. The flame ionization sensor is used to calibrate the gas/air mixture to different gas qualities. The control of the mixing ratio of the gas/air mixture over the modulation range of the gas burner is independent from the flame ionization current.
As mentioned above, EP 2 667 097 A1 discloses a method for operating a gas burner appliance in which the defined mixing ratio of the gas/air mixture is kept constant over the entire modulation range of the gas burner. Only during the calibration mode, the mixing ratio of the gas/air mixture can be changed to compensate for a changing gas quality. However, after a calibration has been executed, the mixing ratio of the gas/air mixture is kept constant over the entire modulation range of the gas burner appliance. The calibration as disclosed by EP 2 667 097 A1, that is used to compensate for a changing gas quality, is performed in a certain subrange of the modulating range of the gas burner close to full-load operation of the same, preferably between 50% (corresponds to a modulation of “2”) and 100% (corresponds to a modulation of “1”) of full burner load operation.